Gracefully handle offline-mode login attempts on online-mode.

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -19,7 +19,7 @@ import com.velocitypowered.proxy.command.ShutdownCommand;
 import com.velocitypowered.proxy.command.VelocityCommand;
 import com.velocitypowered.proxy.config.VelocityConfiguration;
 import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
-import com.velocitypowered.proxy.connection.http.NettyHttpClient;
+import com.velocitypowered.proxy.network.http.NettyHttpClient;
 import com.velocitypowered.proxy.command.VelocityCommandManager;
 import com.velocitypowered.proxy.config.AnnotatedConfig;
 import com.velocitypowered.proxy.messages.VelocityChannelRegistrar;
@@ -39,9 +39,7 @@ import net.kyori.text.serializer.GsonComponentSerializer;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
-import java.io.IOException;
 import java.nio.file.Files;
-import java.nio.file.NoSuchFileException;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.security.KeyPair;

proxy/src/main/java/com/velocitypowered/proxy/connection/client/LoginSessionHandler.java

@@ -101,14 +101,29 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
                                 // The player disconnected after we authenticated them.
                                 return;
                             }
-                            
+
+                            // Go ahead and enable encryption. Once the client sends EncryptionResponse, encryption is
+                            // enabled.
                             try {
                                 inbound.enableEncryption(decryptedSharedSecret);
                             } catch (GeneralSecurityException e) {
                                 throw new RuntimeException(e);
                             }
 
-                            initializePlayer(VelocityServer.GSON.fromJson(profileResponse, GameProfile.class), true);
+                            if (profileResponse.getCode() == 200) {
+                                // All went well, initialize the session.
+                                initializePlayer(VelocityServer.GSON.fromJson(profileResponse.getBody(), GameProfile.class), true);
+                            } else if (profileResponse.getCode() == 204) {
+                                // Apparently an offline-mode user logged onto this online-mode proxy. The client has enabled
+                                // encryption, so we need to do that as well.
+                                logger.warn("An offline-mode client ({} from {}) tried to connect!", login.getUsername(), playerIp);
+                                inbound.closeWith(Disconnect.create(TextComponent.of("This server only accepts connections from online-mode clients.")));
+                            } else {
+                                // Something else went wrong
+                                logger.error("Got an unexpected error code {} whilst contacting Mojang to log in {} ({})",
+                                        profileResponse.getCode(), login.getUsername(), playerIp);
+                                inbound.close();
+                            }
                         }, inbound.getChannel().eventLoop())
                         .exceptionally(exception -> {
                             logger.error("Unable to enable encryption", exception);

proxy/src/main/java/com/velocitypowered/proxy/network/http/NettyHttpClient.java

@@ -1,4 +1,4 @@
-package com.velocitypowered.proxy.connection.http;
+package com.velocitypowered.proxy.network.http;
 
 import com.velocitypowered.proxy.VelocityServer;
 import io.netty.bootstrap.Bootstrap;
@@ -46,7 +46,7 @@ public class NettyHttpClient {
         };
     }
 
-    public CompletableFuture<String> get(URL url) {
+    public CompletableFuture<SimpleHttpResponse> get(URL url) {
         String host = url.getHost();
         int port = url.getPort();
         boolean ssl = url.getProtocol().equals("https");
@@ -54,7 +54,7 @@ public class NettyHttpClient {
             port = ssl ? 443 : 80;
         }
 
-        CompletableFuture<String> reply = new CompletableFuture<>();
+        CompletableFuture<SimpleHttpResponse> reply = new CompletableFuture<>();
         InetSocketAddress address = new InetSocketAddress(host, port);
         poolMap.get(address)
                 .acquire()

proxy/src/main/java/com/velocitypowered/proxy/network/http/SimpleHttpResponse.java

@@ -0,0 +1,27 @@
+package com.velocitypowered.proxy.network.http;
+
+public class SimpleHttpResponse {
+    private final int code;
+    private final String body;
+
+    SimpleHttpResponse(int code, String body) {
+        this.code = code;
+        this.body = body;
+    }
+
+    public int getCode() {
+        return code;
+    }
+
+    public String getBody() {
+        return body;
+    }
+
+    @Override
+    public String toString() {
+        return "SimpleHttpResponse{" +
+                "code=" + code +
+                ", body='" + body + '\'' +
+                '}';
+    }
+}

proxy/src/main/java/com/velocitypowered/proxy/network/http/SimpleHttpResponseCollector.java

@@ -1,4 +1,4 @@
-package com.velocitypowered.proxy.connection.http;
+package com.velocitypowered.proxy.network.http;
 
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.ChannelInboundHandlerAdapter;
@@ -10,10 +10,11 @@ import java.util.concurrent.CompletableFuture;
 
 class SimpleHttpResponseCollector extends ChannelInboundHandlerAdapter {
     private final StringBuilder buffer = new StringBuilder(1024);
-    private final CompletableFuture<String> reply;
+    private final CompletableFuture<SimpleHttpResponse> reply;
+    private int httpCode;
     private boolean canKeepAlive;
 
-    SimpleHttpResponseCollector(CompletableFuture<String> reply) {
+    SimpleHttpResponseCollector(CompletableFuture<SimpleHttpResponse> reply) {
         this.reply = reply;
     }
 
@@ -23,11 +24,7 @@ class SimpleHttpResponseCollector extends ChannelInboundHandlerAdapter {
             if (msg instanceof HttpResponse) {
                 HttpResponse response = (HttpResponse) msg;
                 HttpResponseStatus status = response.status();
-                if (status != HttpResponseStatus.OK) {
-                    reply.completeExceptionally(new RuntimeException("Unexpected status code " + status.code()));
-                    return;
-                }
-
+                this.httpCode = status.code();
                 this.canKeepAlive = HttpUtil.isKeepAlive(response);
             }
 
@@ -38,7 +35,7 @@ class SimpleHttpResponseCollector extends ChannelInboundHandlerAdapter {
                     if (!canKeepAlive) {
                         ctx.close();
                     }
-                    reply.complete(buffer.toString());
+                    reply.complete(new SimpleHttpResponse(httpCode, buffer.toString()));
                 }
             }
         } finally {